This invention relates to a marine propulsion unit and more particularly to an improved apparatus for providing power assistance to the steering and gear shifting of a watercraft or the like.
In many marine applications the watercraft is steered, throttled, and gear shifted by an operator at a remote location. In order to effect steering and gear shifting, the steering device of the watercraft, either the rudder or the outboard drive, and the gear shift lever and throttle of the transmission of an engine, may each be operated by their associated control line.
Steering with control lines is often difficult, as is the case for shifting the gears of the engine due to the resistance encountered. With steering, and especially when turning at higher speeds, the resistance to steering is significant. In gear shifting, if the engine speed is too great during shifting, the gear shift mechanism may be damaged.
One solution to the problem has been the use of a fully automatic hydraulic control scheme. A fully hydraulic system, in the event of failure, causes the loss of all control. The alternative is an intricate backup system, engagable upon main hydraulic failure, to maintain control in the failure mode.
One alternative to the limitations described above involve the use of a power assisted mechanical control system for both the steering and gear shifting. One such system has employed a large hydraulic fluid pump to feed a pair of opposing hydraulic cylinders to achieve steering control. A separate hydraulic pump, is then needed to power the gear shifting action of the marine propulsion unit.
The problems noted in the preceding paragraph can be solved if a single hydraulic pump is employed to operate both of the power assisted controls. However, the size of the hydraulic pump will depend upon the maximum loads of the controlled devices. This can result in the use of large and expensive hydraulic pumps. There is a further need, therefore, to provide a hydraulic assist system wherein the power assisted loads have demands such that a smaller, constant displacement pump may be employed for operating a plurality of power assisted motors.